Wellbores are drilled in the Earth from the surface to one or more subsurface formations typically by rotating a drillbit against the formation. The drill bit is typically suspended in the borehole by a drill string that extends to the surface. In one example, the drill bit may be rotated by rotating the drill string at the surface. Example of surface rotating systems include a rotary table and a top drive. In another example, the drill bit may be driven by a downhole motor, typically referred to as a “mud motor,” which is typically a component in the drill string, located adjacent to the bit.
In a typical drilling system, the drill string defines a flow passage through which drilling fluid, typically referred to as “drilling mud,” is pumped. The mud flows down the drill string to the drill bit, where it exits through jets in the drill bit. The mud then flows up the annulus between the borehole wall and the drill string, carrying drill cuttings to the surface. Through this process, the mud cools the drill bit and cleans the bottom of the borehole from the drill cuttings that are created as the drilling process progresses.
The mud is also weighted with the addition of various compounds so that the hydrostatic pressure in the borehole is higher than the formation pressure, thereby preventing a well blowout in the event a pressurized subsurface pocket is encountered by the drill bit. It is noted that some wells are drilled using a technique called under balanced drilling, where the mud pressure does not quite compensate for the formation pressure.
Most drilling fluids are a fluid that will gel when the fluid is not pumping. This prevents the drill cuttings from falling back down the hole or from collecting on the low side of a deviated well. If mud flow is stopped, the shear stress in the gel must exceed a certain amount to allow the mud to flow again.